DNA methyl adducts (O6-MeG)

DNA methyl adducts are the primary molecular lesions formed by the alkylating agent dacarbazine and its metabolites. Dacarbazine acts as a prodrug, requiring hepatic activation by cytochrome P450 enzymes (CYP1A1, CYP1A2, and CYP2E1) to generate 5-(3-methyltriazen-1-yl)imidazole-4-carboxamide (MTIC), which then spontaneously decomposes into a reactive methyldiazonium cation (PubChem CID 61130). This cation covalently attaches methyl groups to DNA bases, most notably at the O6 and N7 positions of guanine and the N3 position of adenine (StatPearls, "Dacarbazine"). While N7-methylguanine is the most abundant adduct, O6-methylguanine is the most significant regarding cytotoxicity. During DNA replication, O6-methylguanine mispairs with thymine, a lesion recognized by the mismatch repair (MMR) system. The MMR system's inability to find a correct template leads to "futile cycling," resulting in double-strand breaks, G2/M cell cycle arrest, and apoptosis (PubMed: 15155835). The therapeutic efficacy of drugs producing these adducts is heavily influenced by the expression of O6-methylguanine-DNA methyltransferase (MGMT), a repair enzyme that removes the methyl group from the O6 position, thereby conferring drug resistance (NCI Drug Dictionary).

Other names

O6-methylguanine (O6-MeG)N7-methylguanine (N7-MeG)N3-methyladenine (N3-MeA)Methylated DNADNA alkylation lesions

Dacarbazine is a prodrug that is metabolically activated to a methyldiazonium ion, which covalently methylates DNA bases, particularly at the O6 position of guanine. This O6-methylguanine adduct causes base-pairing errors during replication, leading to the activation of the mismatch repair (MMR) system. The resulting futile repair cycles generate double-strand DNA breaks, which trigger G2/M cell cycle arrest and apoptosis (StatPearls; PubMed: 15155835).